I can't tell sometimes items like this might be pouch clasps although I still classify them as menuki especially when they are mantis (thanks for posting by the way). I am not familar with Goto using inlays like that- Goto typically uses gold as its primary inlay. There are some schools that do other inlays although I am not sure about the actual stones in this one being used in their pieces. There is the Hirata school, I think there is another school or two names that are escaping me at the moment.
A little story: There is a Hirata tsuba that has been up for sale and I thought the asking price was crazy until I was able to see the piece better as well as did some reading on the school, discovering that they used different materials and the actual tsuba I was looking at was done with enamals, very nice. From the picture I thought it was just simple normal inlay techniques, but when I was able to see the inlay (or overlay may be a better descriptor) work better, I understood. Here is a little blurb about the school and some examples.http://tsuba.jyuluck-do.com/HirataSchool.htmlhttp://www.silk-road.us/hirata.htmlhttp://www.scholarsresource.com/browse/ ... 2142569339
I don't know enough about the school though to know if your piece fits stylistically as a hirata piece. From my understand Hirata pieces are rare so you don't see them often. I would of course keep it for you if you wanted.
Thanks for posting, nice little piece.